High Pressure High Temperature (HPHT) wells necessitate a requirement to bleed down casing string annuli, to prevent thermal pressure loads from damaging the completion casing program. Thermal expansion of trapped fluid in the casing annuli could otherwise lead to excessive pressure build up causing damage to or failure of the casing completion system.
Annulus bleed down can be readily achieved on surface wellhead applications, as the wellhead housing can be provided with annulus outlets. Subsea wellheads do not have annulus outlets. Each casing string is instead suspended and sealed within the wellhead high pressure housing. No provision is made for communication between each casing string annulus and the wellhead exterior. Assuming that it would be possible to extract annulus fluid as and when required, there is the further problem of disposing of the bled off fluid in an environmentally acceptable way. With the introduction of HPHT completions into the subsea environment, there is a need for subsea wellheads that can facilitate annulus bleed downs.
According to the present invention, a subsea wellhead comprises a monitoring and/or bleed down port extending laterally through a wall of the wellhead housing and having an interior end connected to a well annulus and an exterior end connectable to a jumper for conveying pressure signals and/or expelled annulus fluid to a controls interface.
A preferred embodiment of the invention facilitates the isolation and pressure monitoring of each casing annulus, via a remotely deployable electro/hydraulic control jumper providing a link between the wellhead casing annuli and the subsea production control facility, or a workover control system, as desired. The invention may be used with particular advantage in conjunction with a drill-through horizontal Christmas tree.
The preferred embodiment makes use of three primary components.
1. A modified subsea wellhead housing containing linked annulus ports.
2. A bolt on valve block incorporating independent isolation valves, pressure monitoring equipment and an electro/hydraulic control interface. Alternatively, some or all of these components may be integrated into the wellhead itself.
3. An ROV/diver deployable electro/hydraulic control stab plate jumper to facilitate remote connection between the subsea production control system and the wellhead electro/hydraulic control interface.
Further preferred features of the invention are in the dependent claims and in the following description of an illustrative embodiment made with reference to the drawings.